It is known to detect linear position by providing a linear pattern that encodes a pseudo-random binary sequence (PRBS, also known as an "m-sequence") and then using the window property of such a sequence to effect position detection by detecting a window-length subsequence of the pattern. An example of such a system is described in the article "New Pseudorandom/Natural Code Conversion Method" by E. M. Petriu appearing in Electronic Letters 27th Oct. 1988 Vol.24 No.22.
By "window property" is meant the property that for a PRBS of length (2.sup.k -1), where k is an integer, any subsequence of length k will be unique and therefore uniquely locatable in the sequence; this and other properties of PRBSs are described, for example, in the article "Pseudo-Random Sequences and Arrays" by F. Jessie MacWilliams and Neil J. A. Sloane appearing in the Proceedings of the IEEE, Vol.64, No.12 December 1976.
The prior-art systems utilising the window-property of a PRBS for position sensing are essentially limited to linear position-sensing, although the path followed by the PRBS pattern may, of course, be other than rectilinear (for example, circular or serpentine).
It is an object of the present invention to provide position-sensing apparatus for sensing position over a two-dimensional surface.